Use Formula#system and existing error handling in scons helper

Fixes Homebrew/homebrew#27553.

Library/Homebrew/extend/fileutils.rb

@@ -85,9 +85,7 @@ module FileUtils extend self
   # Run scons using a Homebrew-installed version, instead of whatever
   # is in the user's PATH
   def scons *args
-    scons = Formulary.factory("scons").opt_prefix/'bin/scons'
-    raise "#{scons} is not executable" unless scons.exist? and scons.executable?
-    safe_system scons, *args
+    system Formulary.factory("scons").opt_bin/"scons", *args
   end
 
   def rake *args